This invention relates to a method of forming honeycombed structures from joined cellular segments themselves formed from a ceramic based or other suitable material so as to improve their thermal shock resistance, and in particular to a method of constructing a heat recovery wheel having improved thermal shock resistance and the wheel produced thereby.
Honeycombed structures are used in a variety of applications such as catalytic reactors and heat recovery wheels for conditioning flowing fluids, primarily gases. Such structures consist primarily or entirely of a matrix having a plurality of apertures or hollow, open-ended cells which permit the passage of gases through the structure. Because these structures are subjected to relatively severe thermal shock conditions they are commonly fabricated from ceramic or glass-ceramic materials having very low coefficients of thermal expansion. Other materials (e.g. glass, sintered metal, cermet or other ceramic based materials) could be employed as desired if they were suitable (e.g. sufficient strength, chemical resistance, refractoriness, thermal shock resistance, etc.) with the service conditions encountered.
Heat recovery wheels (also called rotary heat exchangers) are devices for transferring heat from hot gases, generally combustion exhaust, to heat relatively cooler gases, often air to be preheated for combustion. Heat exchange is accomplished by rotating the wheel through simultaneous flows of relatively hot and cold gases. The portion of the wheel's matrix exposed to the gas flows alternately absorbs heat from the hot gas and and releases it to the cold gas. Matrices can be produced by the processes of extrusion or "wrapping" (the building up of corrugated layers). The larger size ceramic wheels needed for efficient industrial heat recovery uses (typically two or more feet in diameter) are most commonly formed by cementing together smaller cellular segments made by the wrap process.
Corning Glass Works has for some time manufactured heat recovery wheels constructed from cemented cellular segments of wrapped glass-ceramic material of a type disclosed in U.S. Pat. No. 3,600,204, having a very low coefficient of thermal expansion (less than 10.times.10.sup.-7 /.degree.C. over the range 0.degree.-1000.degree. C.). This material however is not suitable for all applications because of its susceptibility to attack by hydrogen and sodium ions present in certain exhaust gases. It was discovered in attempting to construct wheels from cordierite materials having a greater resistance to such attack but also a somewhat greater coefficient of thermal expansion (on the order of 20 to 30.times.10.sup.-7 /.degree.C.) that premature cracking consistently occurred in the cement joint areas between the cellular segments, which would continue to propagate until wheel failure occurred.
The previously employed method of constructing such wheels consisted of joining several cellular segments with cement which was applied continuously between the joined cellular segments so as to form solid cement joints across the annular faces of the resulting wheel. This method is now known to be the cause of higher thermally induced stresses in the cement joint areas and adjoining matrix. Thermally induced stresses in these wheels are directly proportional to the magnitudes of both temperature differences and the coefficients of thermal expansion of the materials used. The blockage of gas flow resulting from the continuous cement joints created significant thermal differences across the annular faces and through the axial thickness of the wheel in the joint areas. These stresses were generally not so great as to cause regular cracking under prevalent operating conditions in wheels fabricated from the more stable glass-ceramic material. Also, the old continuous joint cementing method created a rather rigid wheel. Since the level of thermal stresses is also directly proportional to effective modulus of elasticity, this resulted in a more highly stressed wheel. Applicants believe these relationships were not heretofore perceived.